Stretched films with maintained tear resistance and methods for making the same

ABSTRACT

Methods of stretching thermoplastic films in the machine direction include elongating the films in the machine direction without significantly reducing the films&#39; machine-direction tear resistance. In one or more implementations, methods of stretching thermoplastic films include reducing the gauge of the films while substantially maintaining the films&#39; machine-direction tear resistance. The methods can involve uniformly cold stretching the films by stretching the films to a draw of about one hundred and eighty percent, or less, of the films&#39; original length.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application and claims priority toco-pending U.S. Ser. No. 13/218,666, filed on Aug. 26, 2011, which isherein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. The Field of the Invention.

The present invention relates generally to thermoplastic films.Specifically, the invention relates to stretched thermoplastic films,and to methods of stretching thermoplastic films.

2. Background and Relevant Art

Thermoplastic films are a common component in various commercial andconsumer products. For example, grocery bags, trash bags, sacks, andpackaging materials are products that are commonly made fromthermoplastic films. Additionally, feminine hygiene products, babydiapers, adult incontinence products, and many other products includethermoplastic films to one extent or another.

The cost to produce products including thermoplastic film is directlyrelated to the cost of the thermoplastic film. Recently the cost ofthermoplastic materials has risen. In response, many manufacturersattempt to reduce the amount of thermoplastic film in a given product inorder to reduce production costs.

One way manufacturers may attempt to reduce production costs is tostretch the thermoplastic film, thereby increasing its surface area andreducing the amount of thermoplastic film needed to produce a product ofa given size. Common directions of stretching include “machinedirection” and “transverse direction” stretching. As used herein, theterm “machine direction” or “MD” refers to the direction along thelength of the film, or in other words, the direction of the film as thefilm is formed during extrusion and/or coating. As used herein, the term“transverse direction” or “TD” refers to the direction across the filmor perpendicular to the machine direction.

Common ways of stretching film in the machine direction include machinedirection orientation (“MDO”) and incremental stretching. MDO involvesstretching the film between pairs of smooth rollers. Commonly, MDOinvolves running a film through the nips of sequential pairs of smoothrollers. The first pair of rollers rotates at a speed less than that ofthe second pair of rollers. The difference in speed of rotation of thepairs of rollers can cause the film between the pairs of rollers tostretch. The ratio of the roller speeds will roughly determine theamount that the film is stretched. For example, if the first pair ofrollers is rotating at 100 feet per minute (“fpm”) and the second pairof rollers is rotating at 500 fpm, the rollers will stretch the film toroughly five times its original length. MDO stretches the filmcontinuously in the machine direction and is often used to create anoriented film.

Incremental stretching of thermoplastic film, on the other hand,typically involves running the film between grooved or toothed rollers.The grooves or teeth on the rollers intermesh and stretch the film asthe film passes between the rollers. Incremental stretching can stretcha film in many small increments that are evenly spaced across the film.The depth at which the intermeshing teeth engage can control the degreeof stretching. Often, incremental stretching of films is referred to asring rolling.

To MDO stretch a film, manufacturers commonly heat the film to anelevated temperature and stretch the film in the machine direction.Commonly, manufacturers will stretch the thermoplastic film betweenapproximately 300 to 500 percent of the film's original length or more.Unfortunately, stretching thermoplastic films in the machine directionusing conventional methods can weaken the film.

Thermoplastic films have a variety of different strength parameters. Forexample, the tensile strength of a thermoplastic film is the maximumstress that a film can withstand while being stretched before it fails.Tear resistance of a thermoplastic film is the amount of force requiredto propagate or enlarge a tear that has already been created in a film.Still further, impact resistance is the force required to puncture afilm. Stretching thermoplastic films in the machine direction usingconventional methods can reduce the film's machine-direction tearresistance and/or puncture resistance.

The weakening of a film upon stretching may dissuade manufacturers tostretch a film despite the potential material savings. For example, onecommon use of thermoplastic films is as bags for liners in trash orrefuse receptacles. It is desirable to have trash bags with a high tearresistance to help prevent tearing of the trash bag and associatedspilling of the contents during disposal thereof. Another common use ofthermoplastic films is as flexible plastic bags for storing food items.Similar to trash bags, a high tear resistance in food storage bags canhelp prevent tearing of the bags and associated spilling or spoiling offood.

Accordingly, there are a number of considerations to be made inthermoplastic films and manufacturing methods.

BRIEF SUMMARY OF THE INVENTION

Implementations of the present invention solve one or more problems inthe art with apparatus and methods for stretching films in the machinedirection while maintaining the films' machine-direction tearresistance. In particular, one or more implementations of the presentinvention include machine-direction stretched films, and products formedtherewith, that have a maintained machine-direction tear resistance.Additionally, one or more implementations of the present inventioninclude methods of stretching films in the machine direction, whilemaintaining the films' machine-direction tear resistance.

For example, one implementation of a machine-directioncontinuously-stretched film can be formed by stretching an un-stretchedfilm. The machine-direction continuously-stretched film can include athermoplastic material. The stretched film can further include asubstantially uniform gauge that is less than about 98 percent of agauge of the un-stretched film. The stretched film can also include atear resistance that is at least about 75 percent of a first tearresistance of the un-stretched film.

Additionally, one or more implementations of the present inventioninclude a thermoplastic bag. The thermoplastic bag can include first andsecond sidewalls including a machine-direction continuously-stretchedfilm formed by stretching an un-stretched film. The first and secondsidewalls can be joined along three edges. Also, one or more of thefirst and second sidewalls can have a machine direction tear resistancethat is at least about 75 percent of a first machine-direction tearresistance of the un-stretched film. Furthermore, the one or more of thefirst and second sidewalls can have a gauge-by-weight that is less thana first gauge-by-weight of the un-stretched film.

In addition to the forgoing, a method for stretching a film ofthermoplastic material in a machine direction can involve providing afilm of a thermoplastic material with a first machine-direction tearresistance. The method can also involve cold stretching the filmcontinuously in the machine direction at a draw less than about 80percent. The continuously stretched film can have a secondmachine-direction tear resistance that is at least about 75 percent ofthe first machine-direction tear resistance.

Additional features and advantages of exemplary embodiments of thepresent invention will be set forth in the description which follows,and in part will be obvious from the description, or may be learned bythe practice of such exemplary embodiments. The features and advantagesof such embodiments may be realized and obtained by means of theinstruments and combinations particularly pointed out in the appendedclaims. These and other features will become more fully apparent fromthe following description and appended claims, or may be learned by thepractice of such exemplary embodiments as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features of the invention can be obtained, a moreparticular description of the invention briefly described above will berendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It should be noted that thefigures are not drawn to scale, and that elements of similar structureor function are generally represented by like reference numerals forillustrative purposes throughout the figures. Understanding that thesedrawings depict only typical embodiments of the invention and are nottherefore to be considered to be limiting of its scope, the inventionwill be described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 illustrates a schematic diagram of a thermoplastic film beingcontinuously stretched in the machine direction in accordance with oneor more implementations of the present invention;

FIG. 2 illustrates a bag incorporating a machine direction stretchedfilm in accordance with one or more implementations of the presentinvention;

FIG. 3 illustrates another bag incorporating a machine directionstretched film in accordance with one or more implementations of thepresent invention;

FIG. 4 illustrates a schematic diagram of a bag manufacturing process inaccordance with one or more implementations of the present invention;

FIG. 5 illustrates a schematic diagram of another bag manufacturingprocess in accordance with one or more implementations of the presentinvention; and

FIG. 6 illustrates a schematic diagram of yet another bag manufacturingprocess in accordance with one or more implementations of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One or more implementations of the present invention include apparatusand methods for stretching films in the machine direction whilemaintaining the films' machine-direction tear resistance. In particular,one or more implementations of the present invention includemachine-direction stretched films, and products formed therewith, thathave a maintained machine-direction tear resistance. Additionally, oneor more implementations of the present invention include methods ofstretching films in the machine direction, while maintaining the films'machine-direction tear resistance.

Indeed, one or more implementations of the present invention can providethermoplastic films, and products made there from, with reduced gaugeyet maintained tear resistance. Thus, one or more implementations canreduce the material needed to produce a product without compromisingimportant material properties, such as tear resistance. One willappreciate in light of the disclosure herein that such materialreductions can provide significant cost savings.

As alluded to previously, one or more implementations include methods ofcontinuously stretching a film in the machine direction with theunexpected result of maintaining the MD tear resistance of the film. Inparticular, as will be described in greater detail below, one or moreimplementations provide synergistic effects when cold-stretchingthermoplastic films in the machine direction. Indeed, the films of thepresent invention can undergo one or more film stretching processesunder ambient or cold (non-heated) conditions. This differssignificantly from most conventional processes that stretch films underheated conditions. Stretching under ambient or cold conditions inaccordance with one or more implementations can constrain the moleculesin the film so they are not as easily oriented as under heatedconditions. This, in combination with by stretching the films to a drawof about 1.8, or less, of the films' original length can provide theunexpected result of maintaining the MD tear resistance of the film.

Film Materials

As an initial matter, the thermoplastic material of the films of one ormore implementations can include, but are not limited to, thermoplasticpolyolefins, including polyethylene and copolymers thereof andpolypropylene and copolymers thereof. The olefin based polymers caninclude the most common ethylene or propylene based polymers such aspolyethylene, polypropylene, and copolymers such as ethylenevinylacetate (EVA), ethylene methyl acrylate (EMA) and ethylene acrylicacid (EAA), or blends of such polyolefins.

Other examples of polymers suitable for use as films in accordance withthe present invention include elastomeric polymers. Suitable elastomericpolymers may also be biodegradable or environmentally degradable.Suitable elastomeric polymers for the film includepoly(ethylene-butene), poly(ethylene-hexene), poly(ethylene-octene),poly(ethylene-propylene), poly(styrene-butadiene-styrene),poly(styrene-isoprene-styrene), poly(styrene-ethylene-butylene-styrene),poly(ester-ether), poly(ether-amide), poly(ethylene-vinylacetate),poly(ethylene-methylacrylate), poly(ethylene-acrylic acid),poly(ethylene butylacrylate), polyurethane,poly(ethylene-propylene-diene), ethylene-propylene rubber.

The examples and description herein below refer to films formed fromlinear low density polyethylene. The term “linear low densitypolyethylene” (LLDPE) as used herein is defined to mean a copolymer ofethylene and a minor amount of an olefin containing 4 to 10 carbonatoms, having a density of from about 0.910 to about 0.926, and a meltindex (MI) of from about 0.5 to about 10. For example, some examplesherein use an octene comonomer, solution phase LLDPE (MI=1.1; ρ=0.920).Additionally, other examples use a gas phase LLDPE, which is a hexenegas phase LLDPE formulated with slip/AB (MI=1.0; ρ=0.920). One willappreciate that the present invention is not limited to LLDPE, and caninclude “high density polyethylene” (HDPE), “low density polyethylene”(LDPE), and “very low density polyethylene” (VLDPE). Indeed films madefrom any of the previously mentioned thermoplastic materials orcombinations thereof can be suitable for use with the present invention.

Indeed, implementations of the present invention can include anyflexible or pliable thermoplastic material which may be formed or drawninto a web or film. Furthermore, the thermoplastic materials may includea single layer or multiple layers. Examples of multilayered filmssuitable for use with one or more implementations of the presentinvention include coextruded multilayered films, multiple filmscontinuously laminated together, and multiple films partially laminatedtogether. The thermoplastic material may be opaque, transparent,translucent, or tinted. Furthermore, the thermoplastic material may begas permeable or impermeable.

In addition to a thermoplastic material, films of one or moreimplementations of the present invention can also include one or moreadditives. For examples, the films can include pigments, voiding agents,slip agents, anti-block agents, or tackifiers. The pigments can includeTiO₂, or other pigments, that can impart a color and/or opacity to thefilm. Some examples of voiding agents suitable for use in the presentinvention include calcium carbonate, magnesium carbonate, bariumcarbonate, calcium sulfate, magnesium sulfate, barium sulfate, calciumoxide, magnesium oxide, titanium oxide, zinc oxide, aluminum hydroxide,magnesium hydroxide, talc, clay, silica, alumina, mica, glass powder,starch, etc. One will appreciate in light of the disclosure herein thatthe foregoing list of additives are examples of some of the additivesthat may be suitable for use with the present invention.

As used herein, the term “flexible” refers to materials that are capableof being flexed or bent, especially repeatedly, such that they arepliant and yieldable in response to externally applied forces.Accordingly, “flexible” is substantially opposite in meaning to theterms inflexible, rigid, or unyielding. Materials and structures thatare flexible, therefore, may be altered in shape and structure toaccommodate external forces and to conform to the shape of objectsbrought into contact with them without losing their integrity. Inaccordance with further prior art materials, web materials are providedwhich exhibit an “elastic-like” behavior in the direction of appliedstrain without the use of added traditional elastic. As used herein, theterm “elastic-like” describes the behavior of web materials which whensubjected to an applied strain, the web materials extend in thedirection of applied strain, and when the applied strain is released theweb materials return, to a degree, to their pre-strained condition.

In addition to the foregoing, one will appreciate in light of thedisclosure herein that manufacturers may form the films or webs to beused with the present invention using a wide variety of techniques. Forexample, a manufacturer can form the films using conventional flat orcast extrusion or co-extrusion to produce monolayer, bi-layer, ormulti-layer films. Alternatively, a manufacturer can form the filmsusing suitable processes, such as, a blown film process to producemonolayer, bi-layer, or multi-layer films. If desired for a given enduse, the manufacturer can orient the films by trapped bubble,tenterframe, or other suitable process. Additionally, the manufacturercan optionally anneal the films thereafter.

In one or more implementations, the films of the present invention areblown film, or cast film. Blown film and cast film is formed byextrusion. The extruder used can be a conventional one using a die,which will provide the desired gauge. Some useful extruders aredescribed in U.S. Pat. Nos. 4,814,135; 4,857,600; 5,076,988; 5,153,382;each of which are incorporated herein by reference. Examples of variousextruders, which can be used in producing the films to be used with thepresent invention, can be a single screw type modified with a blown filmdie, an air ring, and continuous take off equipment.

In a blown film process, the die can be an upright cylinder with anannular opening. Rollers can pull molten plastic upward away from thedie. An air-ring can cool the film as the film travels upwards. An airoutlet can force compressed air into the center of the extruded annularprofile, creating a bubble. The air can expand the extruded circularcross section by a multiple of the die diameter. This ratio is calledthe “blow-up ratio.” When using a blown film process, the manufacturercan collapse the film to double the plies of the film. Alternatively,the manufacturer can cut and fold the film, or cut and leave the filmunfolded.

As used herein, the term “starting gauge” or “initial gauge” refers tothe average distance between the major surfaces of a film before it isstretched. The films of one or more implementations of the presentinvention can have a starting gauge between about 0.1 mils to about 20mils, suitably from about 0.2 mils to about 4 mils, suitably in therange of about 0.3 mils to about 2 mils, suitably from about 0.6 mils toabout 1.25 mils, suitably from about 0.9 mils to about 1.1 mils,suitably from about 0.3 mils to about 0.7 mils, and suitably from about0.4 mils and about 0.6 mils. Additionally, the starting gauge of filmsof one or more implementations of the present invention may not beuniform. Thus, the starting gauge of films of one or moreimplementations of the present invention may vary along the lengthand/or width of the film.

Referring now to the Figures, FIG. 1 illustrates one exemplary processof continuously stretching a thermoplastic film in the machine directionin accordance with an implementation of the present invention. Inparticular, FIG. 1 illustrates an MD stretching process thatcontinuously stretches an un-stretched film 10 by passing theun-stretched film 10 through sequential pairs of rollers 20, 22 and 24,26. As explained in greater detail below, the MD stretching processprocesses of the present invention can stretch the un-stretched film 10in the machine direction, while substantially maintaining the MD tearresistance of the un-stretched film 10.

As shown by the Figures, the rollers 20, 22, 24, 26 can each have agenerally cylindrical shape. The rollers 20, 22, 24, 26 may be made ofcast and/or machined metal, such as, steel, aluminum, or any othersuitable material. One or more of the rollers may be coated with amaterial such as a rubber or urethane to improve gripping of the filmand to reduce slippage. The rollers 20, 22 of the first set of rollerscan rotate in opposite directions about parallel axes of rotation. Forexample, FIG. 1 illustrates that the first roller 20 can rotate about afirst axis of rotation in a counterclockwise direction. FIG. 1 alsoillustrates that the second roller 22 can rotate about a second axis ofrotation in a clockwise direction. The axes of rotation of the first andsecond rollers 20, 22 can be parallel to the transverse direction andperpendicular to the machine direction.

Along similar lines, the rollers 24, 26 of the second set of rollers canrotate in opposite directions about parallel axes of rotation. Forexample, FIG. 1 illustrates that the third roller 24 can rotate about athird axis of rotation in a clockwise direction. FIG. 1 also illustratesthat the fourth roller 26 can rotate about a fourth axis of rotation ina counterclockwise direction. The axes of rotation of the third andfourth rollers 24, 26 can be parallel to the transverse direction andperpendicular to the machine direction.

The first set of rollers 20, 22 can rotate at a first velocity, whilethe second set of rollers 24, 26 rotate at a second velocity that isgreater than the first velocity. The difference between the firstvelocity and the second velocity can cause the un-stretched film 10 tostretch between the first set of rollers 20, 22 and the second set ofrollers 24, 26 in the machine direction. The ratio of the speed of thesecond set of rollers 24, 26 to the first set of rollers 20, 22 isreferred to herein as the “draw ratio.”

The draw ratio can determine the amount of stretching, or draw, of theun-stretched film 10. For example, a draw ratio of 1.5 (i.e., the secondset of rollers 24, 26 rotating at a speed of 1.5 times the speed of thefirst set of rollers 20, 22) will stretch the un-stretched film 10 to adraw of 1.5. In other words, the un-stretched film 10 will stretch 1.5times its original length in the MD direction.

One or more implementations include draw ratios between about 1.1 andabout 1.8. In other words, one or more implementations includestretching the un-stretched film 10 to a draw of between about 1.1 andabout 1.8. As explained in greater detail below, stretching the film inthe machine direction with a draw ratio of less than about 1.8 undercold (non-heated) conditions can produce the unexpected result ofmaintaining the MD tear resistance of the un-stretched film 10.

Thus, the films of the present invention can undergo one or more filmstretching processes under ambient or cold (non-heated) conditions tostretch the films to a draw of less than about 1.8. This differssignificantly from most conventional processes that stretch films to adraw of greater than 3, and commonly as great as 5, under heatedconditions. Stretching under ambient or cold conditions in accordancewith one or more implementations can constrain the molecules in the filmso they are not as easily oriented as under heated conditions. This, incombination with a draw ratio of less than about 1.8 can provide theunexpected result of maintaining the MD tear resistance of the film.

As shown by FIG. 1, the direction of travel of the un-stretched film 10through the first set rollers 20, 22 is parallel to the machinedirection and perpendicular to the transverse direction. As theun-stretched film 10 passes between the first set of rollers 20, 22 andthe second set of rollers 24, 26, the un-stretched film 10 cancontinuously stretch in the machine direction. Furthermore, in someimplementations, stretching the un-stretched film 10 in the machinedirection can reduce the width of the un-stretched film 10. For example,as the un-stretched film 10 is lengthened in the machine direction, thefilm's width can be reduced in the transverse direction.

For example, FIG. 1 illustrates that the un-stretched film 10 (i.e., thefilm that is yet to pass through the first set of rollers 20, 22 and thesecond set of rollers 24, 26) can have a substantially flat top surface12 and substantially flat bottom surface 14. The un-stretched film 10can have an initial thickness or starting gauge 16 extending between itsmajor surfaces (i.e., the top surface 12 and the bottom surface 14). Inat least one implementation, the starting gauge 16 can be substantiallyuniform along the length of the un-stretched film 10.

For purposes of the present invention, the un-stretched film 10 need nothave an entirely flat top or bottom surfaces. Indeed, the top surface 12and/or the bottom surface 14 can be rough or uneven. Further, thestarting gauge 16 need not be consistent or uniform throughout theentirety of un-stretched film 10. Thus, the starting gauge 16 can varydue to intentional product design, manufacturing defects, tolerances, orother processing inconsistencies.

In any event, FIG. 1 illustrates the first set of rollers 20, 22 and thesecond set of rollers 24, 26 can process the un-stretched film 10 intoan MD continuously-stretched film 30. The MD continuously-stretched film30 can have a substantially flat top surface 32 and substantially flatbottom surface 34. The MD continuously-stretched film 30 can have athickness or gauge 36 extending between its major surfaces (i.e., thetop surface 32 and the bottom surface 34). In at least oneimplementation, the gauge 36 can be substantially uniform along thelength of the MD continuously-stretched film 30.

In one or more implementations, the gauge 36 of the MDcontinuously-stretched film 30 can be less than the starting gauge 16 ofthe un-stretched film 10. In other implementations, the gauge 36 of theMD continuously-stretched film 30 can be approximately equal to thestarting gauge 16 of the un-stretched film 10. In still furtherimplementations, the gauge 36 of the MD continuously-stretched film 30can be greater than the starting gauge 16 of the un-stretched film 10due to rebounding after stretching.

FIG. 1 illustrates that the configuration of the first set of rollers20, 22 and the second set of rollers 24, 26 can cause the un-stretchedfilm 10 to follow an s-shaped path when passing through the first andsecond sets of rollers 20, 22, 24, 26. The s-shaped path can provide thetension between the first set of rollers 20, 22 and the second set ofrollers 24, 26 necessary to cause the un-stretched film 10 to stretchbetween the second roller 22 and the third roller 24. One willappreciate in light of the disclosure herein that the first set ofrollers 20, 22 and the second set of rollers 24, 26 can include anynumber of different configurations.

Thus, the configuration of the first and second sets of rollers 20, 22,24, 26 illustrated by FIG. 1 is only one implementation of a system forcontinuously stretching an un-stretched film 10. For example, inalternative implementations the un-stretched film 10 can pass throughfirst and second sets of nip rollers running at differing speeds.Indeed, many conventional MDO systems may be implemented to continuouslystretch films in accordance with implementations of the presentinvention, so long as such systems are capable of stretching the filmsunder cold conditions to draws of less than about 1.8.

As alluded to earlier, the MD tear resistance of the MDcontinuously-stretched film 30 can be substantially equal to the MD tearresistance of the un-stretched film 10. Indeed, the MD tear resistanceof the MD continuously-stretched film 30 can be substantially equal tothe MD tear resistance of the un-stretched film 10 despite a reductionin gauge. Furthermore, in one or more implementations, the MD tearresistance of the MD continuously-stretched film 30 can be greater thanor equal to the TD tear resistance of the un-stretched film 10. Thefollowing examples present the results of a series of tests performed onthermoplastic films that were continuously stretched in the machinedirection in accordance with one or more implementations of the presentinvention. These examples illustrate the invention claimed herein andshould not be construed to limit in any way the scope of the invention.

Example 1

In a first example, a thermoplastic film was MD continuously stretchedusing a process similar to that described herein above at variousdifferent draw ratios. The film was a LLDPE having a starting gauge of0.60 mils. The first set of rollers was set at a speed of 300 feet perminute, while the second set of rollers varied according to the drawratios indicated in Table I below. The thermoplastic film wascontinuously stretched in the machine direction under cold or ambientconditions at seven different draw ratios. Specifically, the film was MDcontinuously stretched at draw ratios of 1.05, 1.15, 1.20, 1.40, 1.50,1.80, and 2.5. Table I lists the physical properties of these filmsalong with the physical properties of the un-stretched control film(i.e., draw ratio of 1.0).

TABLE I Physical Properties MD Tear TD Tear GBW relative to relativerelative MD un- TD to un- to un- Actual Draw Tear stretched Tearstretched stretched Gauge Ratio (grams) film (%) (grams) film (%) film(%) (mils) 1.0 293 100.0 593 100.0 100.0 0.600 1.05 295 100.7 521 87.9100.1 0.601 1.15 263 89.8 520 87.7 100.1 0.601 1.20 274 93.5 674 113.7102.1 0.612 1.40 261 89.1 690 116.4 96.3 0.578 1.50 286 97.6 720 121.497.2 0.583 1.80 277 94.5 684 115.3 91.9 0.551 2.5 172 58.7 466 78.6 73.80.443

The results from Table I indicate that the MD continuously-stretchedfilm under certain conditions can has a maintained MD tear resistancecompared to the un-stretched film. Specifically, the results from TableI indicate that the MD tear resistance is substantially maintained(i.e., at least about 75 percent of the MD tear resistance of theun-stretched film) at draw ratios up to about 1.8. More particularly,Table I indicates that the MD tear resistance is maintained (i.e., atleast about 90 percent of the MD tear resistance of the un-stretchedfilm) at draw ratios up to about 1.8.

Indeed, Table I indicates that the MD continuously-stretched film canhave an MD tear resistance between about 85 and about 100 percent of theMD tear resistance of the un-stretched film. Table I further illustratesthat the MD continuously-stretched film can include a maintained MD tearresistance despite a reduction in gauge. One will appreciate in light ofthe disclosure herein that this is an unexpected result, as MD tear isexpected to decrease as the gauge of the film decreases.

Additionally, the results from Table I indicate that the MDcontinuously-stretched film under certain conditions can have amaintained or increased TD tear resistance compared to the un-stretchedfilm. Specifically, the results from Table I indicate that the TD tearresistance is substantially maintained or increased at draw ratios up toabout 1.8. Indeed, Table I indicates that the MD continuously-stretchedfilm can have a TD tear resistance between about 87 and about 121percent of the TD tear resistance of the un-stretched film.

Table I further illustrates that the MD tear resistance significantlydrops when the un-stretched film is stretched using draw ratios greaterthan about 1.8. For example, at a draw ratio of about 2.5 the MD tearresistance drops to 58.7 percent of the MD tear resistance of theun-stretched film.

Table I also includes data regarding the gauge-by-weight (“GBW”) of theMD continuously-stretched films. In particular, Table I shows that atdraw ratios of less than about 1.8, the MD tear resistance percentchange relative to un-stretched film is approximately equal to the GBWpercent change relative to un-stretched film. This is in contrast to thedraw ratio of 2.5 in which the MD tear relative to un-stretched film issignificantly less than the GBW relative to un-stretched film.

Example 2

In a second example, a thermoplastic film was MD continuously stretchedusing a process similar to that described herein above at variousdifferent draw ratios. The film was a LLDPE having a starting gauge of1.20 mils. The first set of rollers was set at a speed of 300 feet perminute, while the second set of rollers varied according to the drawratios indicated in Table II below. The thermoplastic film was MDcontinuously stretched under cold or ambient conditions at fourdifferent draw ratios. Specifically, the film was MD continuouslystretched at draw ratios of 1.20, 1.40, 1.60, and 1.80. Table II liststhe physical properties of these films along with the physicalproperties of the un-stretched control film (i.e., draw ratio of 1.0).

TABLE II Physical Properties MD Tear TD Tear GBW relative relativerelative MD to un- TD to un- to un- Actual Draw Tear stretched Tearstretched stretched Gauge Ratio (grams) film (%) (grams) film (%) film(%) (mils) 1 352 100.0 720 100.0 100.0 1.20 1.2 307 87.2 739 102.6 91.31.10 1.4 349 99.1 792 110.0 89.1 1.07 1.6 332 94.3 861 119.6 84.3 1.011.8 351 99.7 998 138.6 79.0 0.95

The results from Table II indicate that the MD continuously-stretchedfilm in accordance with one or more implementations of the presentinvention have a maintained MD tear resistance compared to theun-stretched film. Specifically, the results from Table II indicate thatthe MD tear resistance is maintained (i.e., at least about 90 percent ofthe MD tear resistance of the un-stretched film) at draw ratios up toabout 1.8.

Indeed, Table I indicates that the MD continuously-stretched film canhave an MD tear resistance between about 87 and about 100 percent of theMD tear resistance of the un-stretched film. Table II furtherillustrates that the MD continuously-stretched films can include amaintained MD tear resistance despite a reduction in gauge. One willappreciate in light of the disclosure herein that this is an unexpectedresult, as MD tear is expected to decrease as the gauge of the filmdecreases.

Additionally, the results from Table II indicate that the MDcontinuously-stretched film can have a maintained or increased TD tearresistance compared to the un-stretched film. Specifically, the resultsfrom Table II indicate that the TD tear resistance is substantiallymaintained or increased at draw ratios up to about 1.8. Indeed, Table IIindicates that the MD continuously-stretched film can have a TD tearresistance between about 102 and about 140 percent of the TD tearresistance of the un-stretched film.

One will appreciate in light of the disclosure herein that the MDcontinuously-stretched films with maintained MD tear resistance can formpart of any type of product made from, or incorporating, thermoplasticfilms. For instance, grocery bags, trash bags, sacks, packagingmaterials, feminine hygiene products, baby diapers, adult incontinenceproducts, sanitary napkins, bandages, food storage bags, food storagecontainers, thermal heat wraps, facial masks, wipes, hard surfacecleaners, and many other products can include MD continuously-stretchedfilms 30 with maintained MD tear resistance to one extent or another.Trash bags and food storage bags may be particularly benefited by thefilms of the present invention.

Referring to FIG. 2, in a particular implementation of the presentinvention, the MD continuously-stretched film 30 illustrated in FIG. 1may be incorporated in a bag construction, such as a flexible draw tapebag 40. The flexible draw tape bag 40 can include a bag body 42 havingone or more side walls formed from a piece of MD continuously-stretchedfilm 30 folded upon itself along a bag bottom 44. Side seams 46 and 48can bond the sides of the bag body 42 together to form a semi-enclosedcontainer having an opening 50 along an upper edge 52. The flexible drawtape bag 40 also optionally includes closure means 54 located adjacentto the upper edge 52 for sealing the top of the bag 40 to form afully-enclosed container or vessel. The flexible draw tape bag 40 issuitable for containing and protecting a wide variety of materialsand/or objects. The closure means 54 can comprise flaps, adhesive tapes,a tuck and fold closure, an interlocking closure, a slider closure, azipper closure or other closure structures known to those skilled in theart for closing a bag.

The flexible draw tape bag 40 can require less material to form than anidentical bag formed with an un-stretched film 10 of the samethermoplastic material. Additionally, despite requiring less material,the flexible draw tape bag 40 can have the approximately same MD tearresistance as an identical bag formed with an un-stretched film 10 ofthe same thermoplastic material. The maintained MD tear resistance canhelp prevent the flexible draw tape bag 40 from tearing and losing thecontents.

FIG. 3 illustrates a tie bag 60 incorporating an MDcontinuously-stretched film 30 in accordance with an implementation ofthe present invention. The tie bag 60 can include sides walls formedfrom a piece of MD continuously-stretched film 30 folded upon itselfalong a bag bottom 62. In comparison with the flexible draw tape bag 40of FIG. 2, the tie bag 60 can include flaps along the upper edge 66. Theflaps can allow a user to close off an opening 64 of the tie bag 60.Similar to flexible draw tape bag 40, the tie bag 60 can require lessmaterial to form than an identical bag formed with an un-stretched film10 of the same thermoplastic material. Additionally, despite requiringless material, the tie bag 60 can have the approximately same MD tearresistance as an identical bag formed with an un-stretched film 10 ofthe same thermoplastic material.

As mentioned, the bags 30, 40 can include a bag body having one or moreside walls formed from a piece of MD continuously-stretched film 30. Inone or more implementations, the bag 30, 40 include a first sidewallformed from an MD continuously-stretched film 30. The first side wallcan include a relatively uniform or consistent gauge. In one or moreimplementations, the second, opposing side wall can include one or moreribs, stripes, or otherwise non-planar elements. In other words, thesecond, opposing side wall can include have a gauge that varies. Forexample, the second, opposing side wall can comprise ribs extendingacross the side wall. Such ribs can be formed by a machine-direction,transverse direction, and/or diagonal direction ring rolling process.Thus, the ribs can comprise alternating thicker and thinner strips. Suchribs can extend across the sidewall in the machine direction, thetransverse direction, and/or a diagonal direction. Alternatively, oradditionally, the non-planar elements can comprise raised rib-likeelements formed by a structural elastic like film (SELF) process. Suchraised rib-like elements can be arranged in patterns, such as circles,squares, diamonds, hexagons, or other polygons and shapes.

FIG. 4 illustrates an exemplary embodiment of a high-speed manufacturingprocess 70 for continuously stretching a thermoplastic film in themachine direction and producing a plastic bag there from. According tothe process 70, an un-stretched film 10 is unwound from roll 72 anddirected along a machine direction. The un-stretched film 10 can passabout a first set of rollers 20, 22 and a second set of rollers 24, 26to continuously stretch the un-stretched film 10 in the machinedirection. In one or more implementations, the first set of rollers 20,22 and a second set of rollers 24, 26 are not heated and theun-stretched film 10 is stretched under cold or ambient conditions.

The first and second sets of rollers 20, 22, 24, 26 can have aconstruction similar to that shown in FIG. 1. The first set of rollers20, 22 can rotate at a first velocity, and the second set of rollers 24,26 can rotate at a velocity between about 1.1 and about 1.8 timesgreater than the velocity of the first set of rollers 20, 22. In variousimplementations, motors may be provided that power rotation of therollers 20, 22, 24, 26 in a controlled manner. The velocity differencebetween the first and second sets of the rollers can continuouslystretch the un-stretched film 10 to create an MD continuously-stretchedfilm 30.

During the manufacturing process 70, the MD continuously-stretched film30 can pass through a pair of pinch rollers 74, 76. The pinch rollers74, 76 can grasp the MD continuously-stretched film 30. The pinchrollers 74, 76 may facilitate and accommodate the MDcontinuously-stretched film 30.

A folding operation 78 can fold the MD continuously-stretched film 30 toproduce the sidewalls of the finished bag. The folding operation 78 canfold the MD continuously-stretched film 30 in half along the transversedirection. In particular, the folding operation 78 can move a first edge80 adjacent to the second edge 82, thereby creating a folded edge 84.The folding operation 78 thereby provides a first film half 86 and anadjacent second web half 88. The overall width 100 of the second filmhalf 88 can be half of the width 90 of the pre-folded MDcontinuously-stretched film 30.

To produce the finished bag, the processing equipment may furtherprocess the folded MD continuously-stretched film 30. In particular, adraw tape operation 102 can insert a draw tape 104 into the MDcontinuously-stretched film 30. Furthermore, a sealing operation 106 canform the parallel side edges of the finished bag by forming heat seals108 between adjacent portions of the MD continuously-stretched film 30.The sealing operation 106 can space the heat seals 108 along the foldedMD continuously-stretched film 30. The sealing operation 106 can formthe heat seals 108 using a heating device, such as, a heated knife.

A perforating operation 110 may form a perforation 112 in the heat seals108 using a perforating device, such as, a perforating knife. Theperforations 112 in conjunction with the folded outer edge 84 can defineindividual bags 114 that may be separated from the MDcontinuously-stretched film 30. A roll 116 can wind the MDcontinuously-stretched film 30 embodying the finished bags 114 forpackaging and distribution. For example, the roll 116 may be placed intoa box or bag for sale to a customer.

In still further implementations, the folded MD continuously-stretchedfilm 30 may be cut into individual bags along the heat seals 108 by acutting operation. In another implementation, the folded MDcontinuously-stretched film 30 may be folded one or more times prior tothe cutting operation. In yet another implementation, the side sealingoperation 106 may be combined with the cutting and/or perforationoperations 110.

One will appreciate in light of the disclosure herein that the process70 described in relation to FIG. 4 can be modified to omit or expandedacts, or vary the order of the various acts as desired. For example,FIG. 5 illustrates another manufacturing process 120 for producing aplastic bag from an MD continuously-stretched film 30. The process 120can be similar to process 70 of FIG. 4, except that the un-stretchedfilm 10 is stretched by the first and second sets of rollers 20, 22, 24,26 to form the MD continuously-stretched film 30 after the foldingoperation 78 has folded the un-stretched film 10 in half.

FIG. 6 illustrates yet another manufacturing process 122 for producing abag from an MD continuously-stretched film 30. The process 122 can besimilar to process 70 of FIG. 4, except that the un-stretched film 10 isfolded prior to winding it on the roll 72 a. Thus, in suchimplementations, the un-stretched film 10 unwound from the roll 72 a isalready folded.

Implementations of the present invention can also include methods ofcontinuously stretching a film of thermoplastic material in the machinedirection while at least substantially maintaining the machine-directiontear resistance of the film. The following describes at least oneimplementation of a method with reference to the components and diagramsof FIGS. 1 through 6. Of course, as a preliminary matter, one ofordinary skill in the art will recognize that the methods explained indetail herein can be modified to install a wide variety ofconfigurations using one or more components of the present invention.For example, various acts of the method described can be omitted orexpanded, and the order of the various acts of the method described canbe altered as desired.

For instance, one method in accordance with one or more implementationsof the present invention can include providing a film of a thermoplasticmaterial with a first machine-direction tear resistance. For example,the method can involve extruding an un-stretched film 10 and directingthe un-stretched film 10 into a high speed manufacturing system. Theun-stretched film 10 of thermoplastic material can have a first MD tearresistance. In some implementations, the un-stretched film 10 can have asubstantially uniform starting gauge 16.

The method can also include cold stretching the film continuously in themachine direction at a draw less than about 80 percent. In one or moreimplementations this can involve passing the un-stretched 10sequentially through a first set of rollers 20, 22 rotating at a firstvelocity and a second set of rollers 24, 26 rotating at a secondvelocity. The second velocity can be between about 1.1 and 1.8 timesgreater than the first velocity. The first set of rollers 20, 22 and thesecond set of rollers 24, 26 can continuously stretch the un-stretchedfilm 10 in the machine direction to form an MD continuously-stretchedfilm 30. The MD continuously-stretched film 30 can have a secondmachine-direction tear resistance that is at least about 75 percent ofthe first machine-direction tear resistance of the un-stretched film 10.

Accordingly, FIGS. 1-6 and the corresponding text specifically show,describe, or otherwise provide a number of systems, components,apparatus, and methods for stretching a film in the machine direction.These apparatus and methods can stretch films while at leastsubstantially maintaining the film's MD tear resistance. There areseveral advantages associated with MD continuously stretching athermoplastic film in accordance with one or more implementations of thepresent invention. First, MD continuously a film can reduce the amountof thermoplastic material needed to produce a film of certaindimensions. Manufacturers can decrease the cost of their products ifthey use less thermoplastic material in their products. Depending on theamount that a film is stretched, this cost savings can be significant.

Second, if a thermoplastic film is continuously stretched in the machinedirection in accordance with one or more implementations of the presentinvention, the film's MD tear resistance can be maintained. This findingis unexpected, as it is generally understood that MD stretching athermoplastic film will decrease the film's MD resistance to tear (i.e.,make the film more susceptible to MD tear). Indeed, in one or moreimplementations the MD tear resistance of a thermoplastic film can bemaintained despite the reduction in gauge. In one or moreimplementations, the MD tear resistance of the MD continuously-stretchedfilm 30 can be approximately equal to the MD tear resistance of theun-stretched film 10.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges that come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

We claim:
 1. A method for stretching a film of thermoplastic material ina machine direction while substantially maintaining a machine-directiontear resistance of the film, comprising: providing an un-stretchedblown, linear low-density polyethylene thermoplastic film continuouslyoriented in a machine direction and having a machine-direction tearresistance; and cold stretching the un-stretched film continuously inthe machine direction at a draw less than about 80 percent; wherein thecontinuously stretched film has a machine-direction tear resistance thatis at least about 75 percent of the machine-direction tear resistance ofthe un-stretched film; and a transverse-direction tear resistance thatis increased compared to a transverse-direction tear resistance of theun-stretched film.
 2. The method as recited in claim 1, whereinstretching the film continuously in the machine direction comprises:passing the film sequentially through a first set of rollers rotating ata first velocity and a second set of rollers rotating at a secondvelocity; and the second velocity is between about 1.1 and about 1.8times greater than the first velocity.
 3. The method as recited in claim2, wherein the first set of rollers and the second set of rollers arearranged so the film follows an s-shaped path when passing through boththe first set of rollers and the second set of rollers.
 4. The method asrecited in claim 1, wherein the continuously stretched film has anaverage gauge less than a starting gauge of the film of thethermoplastic material.
 5. The method as recited in claim 1, wherein thetransverse-direction tear resistance of the continuously stretched filmis between 100.1 percent and about 140 percent of thetransverse-direction tear resistance of the un-stretched film.
 6. Themethod as recited in claim 1, further comprising forming thecontinuously stretched film into a trash bag.
 7. The method as recitedin claim 6, further comprising inserting a draw tape into the trash bag.8. The method as recited in claim 1, wherein a ratio of the machinedirection tear resistance of the continuously stretched film to themachine-direction tear resistance of the un-stretched film isapproximately equal to a ratio of the gauge-by-weight of thecontinuously-stretched film to the gauge-by-weight of the un-stretchedfilm.
 9. The method as recited in claim 3, wherein: passing the filmthrough the first set of rollers comprises passing the film counterclockwise about a first roller of the first set of rollers and clockwiseabout a second roller of the first set of rollers; and passing the filmthrough the second set of rollers comprises passing the film clockwiseabout a first roller of the second set of rollers and counter-clockwiseabout a second roller of the second set of rollers.
 10. A method forincreasing area of thermoplastic films without degrading amachine-direction tear resistance of the thermoplastic films,comprising: providing an initial blown, linear low-density polyethylenethermoplastic film continuously oriented in a machine direction andhaving a machine-direction tear resistance; and cold stretching theinitial film continuously in the machine direction at a draw less thanabout 80 percent such that: the continuously stretched film has amachine-direction tear resistance that is at least 75 percent of themachine-direction tear resistance of the initial film; the continuouslystretched film has a transverse-direction tear resistance that is atleast 87% of a transverse-direction tear resistance of the initial film;and a gauge-by-weight that is less than a gauge-by-weight of the initialfilm.
 11. The method as recited in claim 10, wherein themachine-direction tear resistance of the continuously stretched film isbetween about 85 percent and 100 percent of the machine-direction tearresistance of the initial film.
 12. The method as recited in claim 11,wherein the machine-direction tear resistance of the continuouslystretched film is between about 90 percent and 100 percent of themachine-direction tear resistance of the initial film.
 13. The method asrecited in claim 10, wherein the transverse-direction tear resistance ofthe continuously stretched film is between 87 and 140 percent thetransverse-direction tear resistance of the initial film.
 14. The methodas recited in claim 13, wherein the transverse-direction tear resistanceof the continuously stretched film is between 100 and 140 percent of thetransverse-direction tear resistance of the initial film.
 15. The methodas recited in claim 10, wherein the gauge-by-weight of the continuouslystretched film is between about 80 and about 90 percent of thegauge-by-weight of the initial film.
 16. The method as recited in claim10, wherein stretching the film continuously in the machine directioncomprises: passing the film sequentially through a first set of rollersrotating at a first velocity and a second set of rollers rotating at asecond velocity; and the second velocity is between about 1.1 and about1.8 times greater than the first velocity.
 17. The method as recited inclaim 16, wherein the first set of rollers and the second set of rollersare arranged so the film follows an s-shaped path when passing throughboth the first set of rollers and the second set of rollers.
 18. Themethod as recited in claim 17, wherein: passing the film through thefirst set of rollers comprises passing the film counter clockwise abouta first roller of the first set of rollers and clockwise about a secondroller of the first set of rollers; and passing the film through thesecond set of rollers comprises passing the film clockwise about a firstroller of the second set of rollers and counter-clockwise about a secondroller of the second set of rollers.
 19. The method as recited in claim10, further comprising forming the continuously stretched film into atrash bag.
 20. The method as recited in claim 19, further comprisinginserting a draw tape into the trash bag.